The present invention relates to a fingerprint sensing device comprising an array of capacitive sense elements which each include a sense electrode for providing, in combination with an individual portion of an overlying finger, a capacitance, and addressing means connected to the sense elements through first and second sets of address conductors for addressing the sense elements and providing outputs indicative of the capacitances of the sense elements. The invention relates also to a fingerprint recognition system incorporating such a device.
In a known fingerprint sensing device of the above kind, as described in U.S. Pat. No. 5,325,442, the sense elements are arranged in rows and columns and connected to a peripheral addressing circuit via sets of row and column address conductors. Each sense element includes a thin film transistor, TFT, whose drain electrode is connected to the sense electrode and whose gate and source electrode are connected to a row and a column address conductor respectively. All sense elements in a respective row share the same row conductor while all sense elements in a respective column share the same column address conductor. The sense electrodes are covered with dielectric material and together with individual fingerprint portions placed over the dielectric material constitute capacitors. A gating (selection) signal is applied to each row conductor by the addressing circuit in a respective row address period to turn on the TFTs of the sense elements of each row in sequence so that a predetermined potential present on the column address conductors is applied to the sense electrodes to charge the capacitors. The individual capacitances of these capacitors, dependent on the spacing of the fingerprint portions from the sense electrodes as determined by the presence of a ridge of a trough of the fingerprint, are measured by sensing the current which flows in the column conductors during their charging. At the end of a row address period, the TFTs at the row turn off and the next row of sense elements is addressed by the application of a gating signal to the following row conductor. Each row of sense elements is addressed in this manner in turn and the variation in sensed capacitances produced over the array of sense elements by a fingerprint ridge pattern provides an electronic image or representation of the three dimensional form of the fingerprint surface. For consecutive readings of capacitance images, in successive field scan operations, the charge on the sense electrodes is removed, or at least reduced, before the sense elements are addressed again. This is achieved either by incorporating a resistor in each sense element which is connected between the sense electrode and ground, by changing the predetermined voltage applied to column conductors in successive read cycles, or by arranging the addressing circuit to include an intermediate reset cycle between successive read cycles. The provision of a resistor, for example using a doped semiconductor material, is difficult and complicates fabrication of the sense element array, while the other two approaches cause complications to the addressing circuit.
There is currently much interest in fingerprint sensing devices and the use of thin film, large area, technology enables compact devices to be produced at a relatively low-cost compared with conventional optical sensing devices. Although the device of U.S. Pat. No. 5,325,442 has an advantage in this respect, there is a need to reduce the cost still further while maintaining, or improving, performance.